A Factor IX activator has been identified in human polymorphonuclear leukocytes, when the leukocytes in suspension have been subjected to brief sonic disruption. Normal plasma appears to be a necessary cofactor in this activation process, as is calcium ion. We propose in future investigations to demonstrate biochemically that the nature of the activation of Factor IX carried out by these sonicated white blood cells is similar to the activation of Factor IX catalyzed by Factor XIa. In addition, we propose to identify the subcellular location of the Factor IX activator in the polymorphonuclear leukocytes, and begin investigations regarding physiological stimuli which might lead to the release of this activator.